1. Field of the Invention
The present invention relates to the field of flat panel screen displays which are currently used as a replacement to electrode ray tube technology for display purposes and most particularly in the area of portable computers.
2. Prior Art
Flat panel screen displays are designed to display information and/or images on a screen. They have been extensively used in conjunction with computer systems to act as computer display screens. Instead of using the large and heavy cathode ray tube technology, many flat panel displays utilize Liquid Crystal Display (LCD) technology. LC flat panel displays are relatively light weight and have low power consumption. Therefore, the LC flat panel display screen is ideal for use with portable computers where light weight and low power consumption materials are desired.
However, conventional flat panel LC displays have several problems and drawbacks which stem from their structural design. For instance, images formed on the screen within a conventional LCD must be viewed straight on by the computer user by facing the screen directly, see FIG. 1 direction 38 or otherwise known as a 90 degree view angle. When viewing the flat panel screen tilted or from a vertical or horizontal angle, called "off axis" viewing (as shown by direction 39), conventional LCD screens create display distortions because of the optical nature of the display technology. Most often this off axis viewing leads to contrast degradation and color aberrations. Contrast aberrations in black and white and gray scale displays are manifest in color displays as color aberrations. Both are caused by the angular dependency of contrast in LCDs. On some conventional flat panel LC displays the contrast may distort enough to invert the image at some off axis viewing angles where black pixels becomes white and vice-versa. If the LC display contains colors, then off axis viewing will create color aberrations as well as contrast problems. These contrast and color aberrations are caused in part because the image of conventional LCD screens forms within an LC layer, below the screen surface. In the past, LCD flat panel display structures have not been able to adequately and cost effectively solve these problems. As a result, it would be beneficial to cost effectively reduce or remove the off axis viewing problems associated with conventional flat panel LC displays. The present invention offers such a solution.
Another problem with conventional LC displays is parallax which is related to off axis viewing. Parallax is only a problem with touch screens because of the construction of flat panel LC displays; because of the construction of flat panel LC displays, the image that appears to the user's eye is not at the true tangible screen surface position. The difference between the image's observable position and the surface of the screen is called parallax. If a touch screen computer system is used with this type of LC display then it will be difficult to accurately select various on screen items because of this parallax problem. Further, parallax increases with increasing off-axis viewing angle. The image appears to the user's eye to float in the LC layer itself, and this is the parallax problem. The image does not appear to be at the surface of the display and so when such a display is used in a touch screen arrangement the user is required to touch the surface of the display with the expectation that the surface coordinates correlate to the coordinates of the pixels visible within the display. Parallax results when the angle of view is such that the apparent image is not visibly oriented directly under the corresponding point on the surface of the display (refer to FIG. 2C). The illustration shows a target pixel 212 in the LC layer 205 and a corresponding target point 214 on the surface of the touch screen or top glass 203. The view location 201 is at an angle to the normal of the display, and so the intersection of the line of sight 202 (between the observer location and the target pixel) and the top glass is a point some distance from the target point. The distance between the line of the sight/top glass intersection 210 and the target point 214 is the parallax 220 associated with the particular display and particular viewing angle. The parallax is zero for an observer viewing the target pixel "head-on", i.e. such that the line of sight is normal (perpendicular) to the display surface. The invention solves this problem by transmitting the image of the pixel to the surface of the display such that the target point 214 and the apparent image of the pixel 210 are the same. Therefore, it is desired to eliminate the parallax problem with LC displays. The preferred embodiment of the present invention offers such a solution.
Multilayer designs, also called structures, present another problem faced by prior art LCDs. Conventional LC displays are composed of very complex multilayers and require several different and technologically complicated fabrication steps for each layer. The resultant LC display is therefore relatively expensive. The description of these layers of a prior art LC display will be described in detail further below. It would be very beneficial to be able combine one or more of the associated layers of a conventional LC display in order to reduce the fabrication steps, and thereby reduce costs, of the resultant flat panel display. The present invention provides a solution. In prior art designs, LC displays utilizing fiber-optic material layers to reduce parallax and other off axis viewing disorders are relatively expensive because the fiber optic layers were overlaid over, as an after thought, the prior art multi-layer structures of LC displays which is described below. The present invention offers a unique apparatus viably reducing the cost of manufacture of LC displays.
The operation and structure of a prior art flat panel LC display will now be described. Although the structural composition and operation of a conventional LCD device is well known in the prior art, a brief description of the prior art LCD device is presented herewith because it offers a better understanding of the present invention. FIG. 1 illustrates a cross sectional view of a conventional multilayered LC display structure 24. The display structure is multilayered and forms the screen upon which images will be viewed. Image formation occurs within a liquid crystal ("LC") layer 16. However, the image is a result of an optical interaction between several layers including the LC layer. A computer system 9, or other control unit, interfaces with the LC screen through electronic connections 3 to control the creation of images and data thereon. A portion of the structure of the LC screen has been removed so that certain layers are exposed for description. The straight on angle of view for this screen is illustrated by direction 38 and FIG. 1 shows the screen portion from its side cross section view.
The LC display of FIG. 1 includes a light source 8 which provides backlighting. Not all displays have backlighting or are illuminated from the back. In fact many LC displays on the market, such as those in calculators are not backlight. Those in watches generally have some mechanism for activating a backlight, such as a button, when the backlight is necessary to see the image such as when it is dark. Backlights in general improve contrast and viewability of a display, they do, however, draw far more power than the display itself.
A conventional LC display operates as follows. A standard, unpolarized light source 8 emits output light radiation 15 from behind the screen plane to substantially irradiate the entire screen back plane or the back plane may be irradiated by reflecting light incident on the display from the viewer's environment or some combination (e.g. reflective lighting and light source) thereof. Light rays traverse through a first polarizer layer 10 (approximately 1.5 mm thick) which polarizes the incident light rays in a first arbitrary direction 73. This direction is referred to as the zero (0) degree polarization direction. The light next travels though a glass structure layer 12 (approximately 1.1 mm thick) which is transparent and does not effect the polarization character of the first polarized light. The glass layer 12 is used mainly to provide mechanical support for the LCD structure which rests above. The glass structure layer 12 and the first polarizer layer 10 are mechanically secured together.
What is described is a conventional TFT active matrix LCD (TFT stands for Thin Film Transistor). More prevalent in many applications, such as pocket calculators and many lower cost LCD computer displays, is the directly addressed, or nematic, displays such as Sharp's "Super Twist" and "Double Super Twist" displays. The invention is equally applicable to such displays. The major difference between active and passive displays are that passive displays do not have a transistor associated with and located with each pixel of the display. The resultant simplification of the manufacturing process makes passive displays much less expensive to produce. However, the TFT solves many problems encountered when attempting to control a large matrix of pixels as one might encounter in a computer display (where typically one finds 640.times.400 or more pixels compared to a calculator which might have 60 or 70 total pixel elements), and so TFT displays generally exhibit much higher quality images with better contrast. It is extremely difficult to control a passive matrix display with the accuracy required to create a gray scale image, where each pixel might demonstrate any one of 2 to 256 different levels of contrast, therefore most gray scale and most color displays are active matrix displays. Contrast aberrations associated with off axis viewing are particularly problematic in passively addressed displays; this invention would dramatically improve the viewability of passive matrix displays and would therefore allow their use in a much wider range of products where before they could not be selected because of poor viewability, even considering their significantly lower cost.
Referring still to FIG. 1, the polarized light traverses through an active transistor and output electrode matrix layer 14 (about 50 nm thick) and the LC layer 16 which contains MCLC 133 or 2,3-dicyano phenyl from Merck (approximately 4-10 microns thick) and, which provides the control and structure required to form images. All images created by this flat panel LCD are composed of pixels, or "dots." Each pixel has at least one corresponding transistor and corresponding electrode from the control layer 14. In other words, each transistor is responsible for creating a viewable "pixel" on the viewing screen. A number of localized active transistors may thus create an image. Each transistor is opaque. The ratio of the area of the transistors and the associated connections to the area of the associated electrode is a major factor in the brightness of the display. The transistors are shown in the cut away cross section of layer 14 and are illustrated by the dark dots 32 at each intersection line. The lines represent the control lines for the transistors; the source lines are illustrated by lines 36 running vertically and the gate lines are illustrated by lines 34 running horizontally. To activate a particular transistor 32, its identifying gate lines 34 and source lines 36 are activated. Each transistor 32 also has a corresponding output electrode 30 associated with it and attached to the transistor drain. The output electrodes are insulated from the common electrode 18 by the high dielectric properties of the LC layer, 16; they therefore act as capacitors. The output electrodes 30 are also transparent and will charge up upon transistor activation to create an electric field. Not all of the output electrodes are shown for simplification. Only a few electrodes 30 are shown and shaded, however, it is appreciated that each transistor has its own output electrode. A holding capacitor, not shown, may be fabricated along with the TFT to maintain this charged state longer than the capacitance of the output electrode alone. The characteristic discharge time is the time it takes for the charge on the output electrode to decay enough that the contrast of the corresponding pixel is impaired. The charge on the output electrode must be refreshed before this decay is visible or the display will appear to "flicker". The output electrodes 30 are made of an indium tin oxide and are conductors while maintaining their transparent properties.
If the computer 9 or control unit identifies a particular transistor 32 and activates such, the corresponding output electrode is thereby charged and creates a localized charge between the electrode plate 30, and the common electrode 18. This charge will effect the liquid crystal layer 16 as will be described below.
Referring to FIG. 1, the first polarized light polarized by the polarizer 10 and after passing through the control layer 14, next passes through the liquid crystal layer 16 (approximately 4-10 .mu.m thick). This is a layer of liquid crystal material such as Merck MCLC 133. The liquid is secured on the sides of the layer structure by an epoxy or other material, not shown, to prevent it from flowing out from the sides. The spacing of the layer is maintained by small glass beads or fibers of diameter equal to the thickness of the layer. The horizontal spacing of the beads or fibers is sparse enough to insure that the optical properties of the display are not compromised while insuring the mechanical stability of the structure. Below the liquid crystal layer 16 (shown as 44 in FIG. 1) lacquer is applied to the control layer and rubbed with velvet to create very fine parallel markings in an arbitrary but unidirectional fashion. Above the liquid crystal layer 16 a lacquer 46 is also applied and also rubbed with velvet to create similar fine parallel markings which are perpendicular to the markings on side 44. In the absence of any electric field in the LC layer, these markings will be used to mechanically hold the LC molecules in place. Within the LC layer 16, light rays are either free to pass through unchanged or their polarization direction is rotated 90 degrees, depending on the arrangement of the crystal molecule formation in a manner described below.
Light passes, either changed or unchanged, through the liquid crystal layer 16 to the next layer which is the common electrode layer 18 (approximately 50-60 nm thick). This layer creates a reference voltage for use with each of the independent output electrodes 30 so that an electric field may be produced there between. This common electrode is also transparent and made of a similar material as that of the output electrode 30. The liquid crystal layer 16 is mechanically coupled to, and sandwiched between, both the control layer 14 and the common electrode layer 18. The interaction between these layers will be described later. In color flat panel LC displays, thin film color filters 40 are placed between the common electrode layer 18 and the top glass layer 20. The color filter layer (2 .mu.m thick), 40, allows the development of color images on the display screen. On top of, and mechanically coupled to, the color filter layer 40 is another glass support layer 20 (approximately 1.1-3.0 mm thick but this thickness depends on size and use of LCD). This layer is transparent and also helps to support the LC display from mechanical shock and breakage.
Referring still to FIG. 1, the glass layer 20 is coupled to a second polarizing layer 22 (approximately 1.5 mm thick) which is called the analyzer 22. The analyzer layer 22 polarizes light in a direction 72 perpendicular to that of the direction 73 of the first polarizer 10. Therefore, if the light is not altered in some way, once it becomes polarized in one direction by the first polarizer 10, it will not pass through the analyzer layer since orthogonal polarization completely blocks out the light rays. No light will be seen from view angle 38 and a resultant dark spot forms in the display.
The operation and structure of the LC layer has been hypothesized and is well used in practice in the prior art and described as follows for a clear understanding of the present invention. The first polarized light rays are altered in the liquid crystal layer 16 as a result of the alignment of the liquid crystal molecules 50 as shown in FIG. 2. FIG. 2 illustrates the interaction of the transistor and output electrode control layer 14, the LC layer 16, and the common electrode layer 18. The liquid crystal molecules are shown in FIG. 2(A) in their suspension state when no electric field is present. The long molecules 50 naturally settle into the finely etched groves 44 of the bottom of the LC layer 16. Due to a particular natural arrangement phenomena, not entirely understood by science, the molecules arrange themselves in a spiral staircase fashion 51. The stair case rises until the molecules settle into the upper etched groves 46 which run perpendicular to the lower groves 44. These grove patterns serve to mechanically anchor the spiral staircase molecules and initiate the formation of the staircase pattern.
Referring to prior art FIG. 2(B), application of an electric potential between the TFT and the common ground creates an electric field the direction of which is illustrated by 55. Upon application of an electric field 55 through the LC layer 16, the LC molecules are forced from their staircase arrangement into a vertical arrangement 53 as shown. The molecules line up along the electric field lines 55 from below to above. The unique bi-polar anisotropic optical/electrical behavior of the liquid crystal molecules (such as those supplied by Merck under the name MCLC 133) causes them to experience a torsional force as in interaction between their polar electrical structure and the applied electric field. This torque causes the molecules to rotate in such a way as to align the optical axis of the molecule vertically, as is illustrated by 53. The electric field 55 is generated by the charge interaction of the various output electrodes 30 and the common electrode layer 18 which surround the LC layer. The common electrode is continuous and generally connected to ground. It would be considered to be the minus (-) terminal of a simple DC electric circuit. The output electrode is charged positively or negatively or in an alternating manner with respect to the common electrode plane. When a positive charge settles on one of the output electrodes 30, it will set up a localized electric field through a localized portion of sandwiched LC layer to vertically align a portion of the LC crystals in the layer 16 that are bounded by the area of the charged electrode 30 and the common electrode layer 18. In this fashion, certain portions of the LC may be in staircase formation 51 (since no charge is on the underlying electrode in that portion) while other portions of the LC may be in vertical formation 53, depending on the charge status of each transistor 32 and its associated output electrode 30. When the potential between the TFT and the common electrode is brought to zero the molecules 50 will return to their staircase arrangement 51. It can be appreciated that liquid crystal molecules can be selected and placed in the proper environment wherein they arrange themselves in staircase formation upon application of an electric field and arrange themselves vertically when no field is present, just opposite to the description of the preferred embodiment herein. It would be obvious to make such a differentiation.
When the LC material has its molecules oriented in the staircase fashion 51 it is appreciated that this is a three dimensional structure, and that within each plane of molecules parallel to the layers of the sandwich structure all of the LC molecules have the same orientation it should be appreciated that, for reasons beyond the scope of this document, each molecule of liquid crystal (such as Merck MCLC 133) is optically anisotropic in as much as it is transparent to light oscillating transverse to it's length and opaque to light oscillating parallel to its length. As a combination of this anisotropy and of the overall alignment of the liquid crystal in the staircase arrangement, each molecular layer of LC material acts as a polarizing layer, each layer polarizing light in a direction slightly rotated from the one above it. It is well known that polarizing layers partially rotated one from another are partially transparent through the sum of their polarizations, and that the net polarization of light passing through more than one polarizer is in the direction of the last polarizer through which it has passed. As light passes through each molecular layer of LC material arranged in the stair-step fashion, its polarization is rotated by some small angle. After passing through effective polarizers, each a single molecular layer thick, and each rotating the polarization of the light by 90/n.degree., the light is rotated 90.degree., and has suffered nominal attenuation. It will then freely pass through the last polarizer.
As a result of the crystal molecule formation, polarized light traversing through the LC material, which has its molecules aligned in a staircase fashion 51, is optically rotated by the molecule formation. Light in the zero degree direction of polarization which passes through the staircase formation 51 becomes rotated by 90 degrees. And first polarized light passing through the vertical formation 53 remains unchanged in its zero degree direction of polarization.
As seen by prior art FIGS. 2(A) and 2(B), first polarized light that has been rotated by the crystal molecule staircase has been changed from the zero degree direction 73 to the 90 degree direction 72 of polarization. Therefore, the rotated polarized light rays will freely pass through the analyzer layer 22 which polarizes light to the 90 degree direction by restricting light in the zero degree direction. Thus, light passing through the LC layer having a staircase formation will eventually be seen by the viewer 38 as a "white pixel" on the display. And, first polarized light rays passing through the vertical arrangement of molecules 53 of layer 16 is not rotated and will be fully absorbed at the analyzer layer 22. Light passing through the vertical arrangement 53 will not been seen by the view 38 and will therefore represent a "black pixel" on the display. Thus, when the transistor electrode 30 is charged, molecules 50 located in line between the transistor electrode 30 and the common electrode are aligned vertical and a black pixel is formed on the display screen. When the charge on the transistor electrode 30 is dissipated, the molecules 50 located in line between the transistor electrode 30 and the common electrode are in staircase formation and a white pixel is formed on the display screen.
Using the transistor electrode matrix, and the generation of white or black pixels through the rotation of polarized light in LC layer, images composed of pixels can be formed on the flat panel screen 24. It should be noted that although many layers are required to form the image, some are structural and some are circuitry, the viewer of angle 38, views the image as optically within the LC layer. The image forms optically within the position of the LC layer 16.
The reasons for the problems mentioned previously regarding conventional flat panel LC displays can now be discussed. Because the resultant pixel image is optically formed within the LC layer 16, and not within the outer analyzer layer 22, the image is viewed through at least four layers: the LC layer 16, the common electrode 18, the glass support 20 and the analyzer 22. In color flat panel LCDs, the color filter layer 40 may represent a fifth layer. Often many conventional LC displays contain an external transparent layer of silicon dioxide about (0.5-1.0) mm thick resting on top of the analyzer layer for protection. In this case, the image is viewed through at least five layers. The difference between the image position and the surface of the flat panel display is called the "image depth." Large image depth contributes to the problem of parallax. The larger the image depth the worse the parallax. For instance, when the flat panel screens are viewed off axis (direction 39), the optical position of the image is different from the tangible position on the screen surface. The apparent position of the pixel is not the same as the position on the surface of the screen vertically correspondent with the electrode grid below responsible for creating the apparent image. This is true because the screen tilt and image depth forces the image to appear in a screen location which is not directly above the image and thus difficult to touch when using thick touch sensitive screens and is therefore difficult to target when the display is used in a touch sensitive configuration.
This problem is particularly acute when a touch sensitive screen is used in a computer system where the user of the system provides inputs to the system by pointing to (and hence touching) objects displayed on the screen. The image depth is enlarged when touch sensitive flat panel touch screens are employed because a special thick touch sensitive layer rests on top of the analyzer representing a possible sixth layer. The obvious answer to this problem is to make the layers, through which the image must pass, sufficiently thin to reduce the image depth. However, structural support is required to protect the fragile LC layer 16 (mechanical pressure on the LC layer can modify the alignment of the LC molecules of the structure and can even permanently damage the LC structure) and hence a sufficient thickness of at least layer 20 is required. It has been found that threshold thickness values thick enough to protect the LC layer also create parallax problems. Another suggestion to reduce the image depth in the past has been to remove the glass layer 20 and place the analyzer 22 on top of the common electrode layer 18. However, this does not operate effectively because the analyzer (and polarizer) are plastic. They therefore do not generally survive the process of applying metallization (the conductive layers which make up the electrode grid and the common electrode). More importantly is the problem of the necessary precision with which the gap in the LC material must be maintained. Plastic films, which polarizers are not made with the process herein, do not have the dimensional stability to be used as a top glass, bottom glass, or for any of the structural elements required in LCD manufacture.
In one prior art touch screen configuration a conductive layer is placed on a glass plate which is added on top of the LCD structure to sense the position of a finger or pen. This conductive layer cannot be reliably applied to a plastic layer, hence the requirement for an additional glass layer, which adds to the parallax problem. The invention herein could be used to remedy the problems associated with the before described touch screen configuration as the conductive layer could be applied directly to the glass plate of the invention to be described later. This would alleviate all parallax problems and simplify the construction of the display.
The present invention solves this parallax problem by providing a new layer structure which effectively brings the resultant image up to the outer surface of the new screen display. Therefore, no parallax problems remain because the image depth approaches zero. The new multilayer structure also provides adequate protection to the LC layer.
A second issue is the use of resistive sheet touch screen technology on top of a prior art LCD. Current LCD manufacture requires that an additional glass layer be applied over the completed LCD, above the analyzer layer, on which a moderately conductive (100-500 .OMEGA./square) transparent layer is applied in a similar fashion and of similar characteristics to the common electrode. This layer is used in conjunction with appropriate sensing technology to locate the position of a finger or other conductive object which is in electrical contact at some point with the conductive surface of the touch screen. The requirements of the substrate of such a screen are similar to the requirements of the top glass in that the substrate must withstand the heat of processing required to apply the conductive layer, and that instead of the primacy of dimensional stability in importance, wear resistance is important. Plastic, as is commonly well understood is far less wear resistant than glass, and as such attempts at plastic substrate touch screens do not survive use. For these reasons the structure of a resistive sheet sensing technology LCD assembly cannot be simplified in prior art by placing the conductive layer required for sensing on the analyzer layer, as it is made of plastic. The current invention could be advantageously used in such an assembly by applying the conductive layer required for sensing directly to the front surface of the fiber-optic faceplate, which is glass. Appropriate sensing technology is, for example, manufactured by Micro Touch Systems, Inc., 55 Jonspin Road, Wilmington, Mass. 01887, 508-1694-9900, Fax 508 694-9980.
Another problem with LCDs, particularly super-twist and double super-twist passive matrix displays, to which the herein described invention could equally be applied, is contrast aberrations evident at off-primary-axis viewing angles, indicated as 39. It can be appreciated that the polarization properties of the LC material are determined by their physical alignment to the polarizer and analyzer layers as well as the viewer. It can also be clearly appreciated that when the viewing angle is off-axis, 39, not on-axis, 38, the physical alignment between the various layers and the viewer is changed. This change is manifested in contrast aberrations, frequently so severe at angles greater than a few tens of degrees as to completely invert the image such that those pixels intended to be seen as light appear dark and vice-versa. The problem is particularly apparent in color LCD configurations where the observed color is created by a balance of intensities created by three pixels, one red, one blue, one green. In general the contrast aberrations resulting from off axis viewing are not uniform pixel to pixel and therefore the observed colors in a color LCD very significantly from viewing position to viewing position.
One prior art suggestion has been a layer arrangement that uses a complicated set of (2-4) thin film compensation (.+-.100 .mu.m per film) layers between the analyzer and the top glass in the LCD structure to compensate for the polarization differences resulting when the image rays pass through the LCD structure at various angles to the viewing position. The film compensation layers serve to compensate for variations in the degree of polarization as a function of angle of view. The functional properties of the films used (also known as retardation films) are the birefringence (on the order of 0.004) and the retardation (on the order of 300-500 nm). A description of the physics that underlie the function of the compensation films is beyond the scope of this application; however, the result of the application of retardation films is an increase in the complexity of the LCD structure without fully alleviating contrast problems associated with off-axis viewing.
As seen by FIG. 3, another prior art system attempts to solve the problems associated with off axis viewing by placing a fiber-optic layer 42 on top of the analyzer 22 creating a new structure 25. The physical properties of the fiber-optic layer are such that the image formed within the LC layer is optically resolved onto the surface of the fiber-optic layer. The fiber-optic glass layer 42 contains millions of two ended tiny glass rods standing up on end and fused side by side mechanically together. The rods 48 may be viewed as a collection of fibers or "light tubes." Each light tube is capable of taking an image resolved on one end and optically resolving it at the other end. The image from the LC layer is "pixalated" by these rods when transferred from one rod end to the other thus transferred from the bottom surface of the fiber-optic layer to the top surface. By pixalation it is meant that the details of the underlying image, in this case the image formed in the LC layer, are integrated into each rod. The image data within each rod is scattered such that the rod carries only light of a certain intensity and color. Taken together the rods form an image in much the same way as the LCD itself, but generally on a much smaller scale. Each pixel of the image is transferred by many microscopic fiber-optic glass rods 48. Optically, the image generated in the LC layer appears on the exterior surface of the fiber-optic layer, i.e., as if printed there. The fiber rods, 48, conduct light via an effect known as total internal reflection. Functionally, the rods are small enough that light entering the fibers does so at an angle less than a certain critical angle, known as the Brewster angle, and determined by the ratio of the index of refraction of the core of the fiber and the index of refraction of it's surface or the index of refraction of the medium of it's cladding, if a cladding is used. Once received into the fiber, light bounces from side to side of the fiber. The diameter of the fiber being small enough that the light cannot strike the side of the fiber at an angle greater than some minimum significantly smaller than the brewster angle. As a result the light is transmitted down the fiber by a series of nearly innumerable reflections, each one entirely without loss. It should be noted that since each rod is vertically aligned with the LC layer, it views the LC layer from angle 38. Once the image is transmitted to the surface of the fiber-optic plate, it is viewed with axial dependance of the fiber-optic plate, not the LCD. The off-axis viewing characteristics of fiber-optic materials can be made to be far superior to those of any LCD not using a fiber optic structure.
However, fiber-optic layers have an inherent limitation when used in prior art flat panel LCD screen structures 25. Fiber-optic materials have a very small focal length, on the order of a few hundreds of an inch in most cases. The apparent image floats within the LC layer, 16, and since the thickness of the various required layers between the LC layer, 16, and the fiber-optic layer, 42, is typically greater than the focal distance of the fibers, the result is an entirely unfocused image, generally utterly illegible.
There have been several suggestions in the prior art to reduce the thickness of intervening layers, however they are inadequate as described as follows. In an effort to reduce image depth, the fiber-optic layer 42 cannot be placed directly above the common electrode 18 because the analyzer 22 is required (above the LC layer) to polarize the light. Also, a structural support glass 20 is recommended to protect and support the LCD structure. This glass layer 20 (above the LC layer) and analyzer layer 22 add to the image depth and thus add to the focus problem. In prior art, the analyzer layer, 22, is manufactured from PET [poly-ethylene-trichloride], and so neither the common layer, 18, nor the color filter layer, 40, can be applied to it as processing temperatures required are too high, also, the dimensional stability of PET is too low to maintain the working gap distance required in the LC layer, 16. It has also been suggested to place the fiber-optic layer 42 beneath the analyzer 22 to rid the problems of parallax and off axis contrast degradation. However, the fiber-optic layer will scatter the light polarization which was carefully generated by the LC layer 16 as the light passes through the fiber-optic layer 42. Once scattered, the image will never form even if it passes through the analyzer 22.
The present invention eliminates off axis viewing problems by allowing the image to form at the exterior surface of layers of a new flat panel display structure. Since the image is viewed on the surface of the new exterior layer, no off axis viewing problems result. A specially designed layer is advantageously utilized by the present invention which replaces three layers: the top glass layer 20; analyzer layer 22; and fiber-optic layer 42.
Another problem with the conventional structure of prior art LCDs is the general requirement that the analyzer layer, 22, be the top most layer. As this layer is, in prior art, manufactured from PET, it is relatively soft and therefore subject to damage and wear from normal use. As a solution to this problem in those cases where necessary, such as portable computers, the analyzer layer, 22, is covered with a coating of SiO.sub.2 (silicon dioxide), which is very hard, to minimize the risk of accidental damage. This process adds to the complexity and expense of finished product. Another solution is to add yet another glass layer above the completed sandwich to protect the fragile analyzer. This solution also adds to the complexity and cost of manufacture as well as adding to the weight of the assembly.
Another problem of conventional flat panel displays is their multiple layer structure. This tends to be very expensive and difficult to construct. The present invention effectively eliminates the need for: (1) the top glass layer 20; (2) the analyzer layer 22; (3) the fiber-optic top layer 42; and (4) any silicon dioxide protective layer. In replacement thereof the present invention utilizes a single polarizing fiber-optic layer.
It is therefore an object of the present invention to create a new and improved flat panel display utilizing liquid crystal technology that solves the off axis contrast problems and color aberrations associated with off axis viewing. It is another object of the present invention to create a flat panel display that eliminates the parallax problem that becomes a major problem in touch screen applications. It is another object of the present invention to reduce the number of layers required to fabricate a conventional LC display and thus reduce the end product fabrication costs, and assembly weight.